stash: pass pathspec as pointer

stash: pass pathspec as pointer

Passing the pathspec by value is potentially confusing, as the copy is
only a shallow copy, so save the overhead of the copy, and pass the
pathspec struct as a pointer.

In addition use copy_pathspec to copy the pathspec into
rev.prune_data, so the copy is a proper deep copy, and owned by the
revision API, as that's what the API expects.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>